Rubber articles that utilize silica as a reinforcing filler, especially tires with tread formulations that contain silica, exhibit reduced hysteresis and better rolling resistance, wet traction, and snow traction when compared to tire treads that contain carbon black as the only filler.
In order to improve the dispersion and processing of silica filled rubber stocks, it has been well known in the art to add silica coupling agents having a moiety (e.g., a silyl group) reactive with the silica surface, and a moiety (e.g., a mercapto, amino, vinyl, epoxy or sulfur group) that binds to the elastomer. Thus, the silica coupler provides a chemical link between the silica filler and the elastomer. Well known examples of such silica coupling agents are organosilane polysulfides, such as bis-(trialkoxysilylorgano) polysulfides, and blocked mercaptosilanes.
However, the use of organosilane polysulfides can lead to increased compound viscosity when mixed at elevated temperatures. In addition, silica stocks that utilize organosilane polysulfides typically have to be processed and formed within a few days after mixing due to an increase in compound viscosity that occurs upon aging. This causes manufacturing inefficiencies.
Blocked mercaptosilanes can offer reduced compound viscosity. As a result, fewer mixing stages may be necessary to achieve the desired compound viscosity. Also, blocked mercaptosilanes offer better stabilization of compound viscosity over time, allowing silica stocks to be stored for longer periods before being processed and formed into a final product.
Although blocked mercaptosilanes offer processing advantages, their cost is substantially high. Furthermore, the level of blocked mercaptosilanes used is typically much higher in comparison to organosilane polysulfides due to a decreased coupling yield, which is caused by the reduced number of alkoxy groups present to react with the silica surface and the reduced number of sulfur atoms to react with the elastomer. The higher dosage amount further exacerbates the cost of blocked mercaptosilanes. Therefore, there remains a need to achieve the processing advantages blocked mercaptosilanes offer at a reduced cost.